The present invention relates to measuring the flow rate of intake gas, particularly to a gas-cooled hot-electrical-resistance type gas flow meter which is mounted in an intake line to measure the flow rate of the intake gas by detecting a variation in electrical resistance value of a hot electrical resistance which is cooled by the intake gas.
In ordinary automobiles, the minimum mass flow rate Qi is 10 to 15 Kg/h during idling of the engine and the maximum mass flow rate Qm is 500 to 600 Kg/h on a critical rotational speed of the engine when a displacement of the engine is 4000 c.c.; while the minimum mass flow rate Qi is 5 to 10 Kg/h on idling of the engine and the maximum mass flow rate Qm is 300 to 400 Kg/h on the critical rotational speed of the engine when the displacement of the engine is 2000 c.c.. Therefore, the ratio of the maximum mass flow rate to the minimum mass flow rate (a dynamic range) Qm/Qi is 60 to 80. In the future, the maximum mass flow rate will be increased for purposes of making the rotational speed and output power of the mobile engine higher, while the minimum mass flow rate will be kept at the present level, so that the dynamic range will reach 150.
The flow speed of intake air in an intake line, that is, the flow speed thereof at a gas-cooled hot-electrical-resistance type gas flow meter depends on the cross-sectional area of the main passage (the diameter of the intake line) and so forth and is limited within 0.5 m/s to 50 m/s over a whole range of the flow rate at the present time. The reason for restraining a further increase of the flow speed is that the higher the flow speed of intake air is, the larger will be deterioration (change in characteristics) by extremely small dust which cannot be filtered off by air cleaner, which dust sticks on the gas-cooled hot-electrical-resistance device with the passage of time, as explained on page 26 and FIG. 15 in SAE Paper 840137, 1984. An increase of the dynamic range, that is, an increase of the maximum flow rate causes an increase of the maximum flow speed and accelerates the sticking of the dust on the gas-cooled hot-electrical-resistance.
In a conventional hot-wire type gas flow meter as shown in Publication of Japanese Patent Application Laid-open No. Shou 54-76182, the temperature of the hot wire is increased to more than the normal operational temperature thereof so that the dust on the hot wire is burned out and the cleanliness of the hot wire is maintained. But, since the dust includes calcium, the dust sticks securely on the hot wire even with the temperature increase of the hot wire and the deterioration of the characteristics thereof is made large. In another conventional hot-wire type gas flow meter as shown in Publication of Japanese Patent Application Laid-open No. Shou 59-190624, an obstacle is arranged at an upstream side of the hot wire so that the dust sticks on the obstacle and the dust is prevented from sticking on the hot wire. But, since the flow speed of air at the hot wire is decreased by the obstacle, the sensitivity of the hot wire is decreased when the flow rate of intake air is low. And, since the obstacle generates a disturbance or eddy in the intake air, the output noise of the hot wire is increased.
In another conventional hot-wire type gas flow meter as shown in Publication of Japanese Patent Application Laid-open No. Shou 55-66716, a straight tube receiving the hot wire is arranged in a main passage. In this structure, the flow speed in the tube (on the hot wire) is substantially equal to the flow speed in the main passage surrounding the tube. Therefore, in order to keep the maximum flow speed within a desirable degree when the maximum flow rate of intake air is increased, the cross-sectional area, that is, the diameter of the passage must be increased with an increase in space receiving a duct system. And, in this case, since the minimum flow rate of intake air is not increased, the flow speed is decreased to less than a desirable degree so that the sensitivity of the hot wire is decreased and noise protection is needed.
In another conventional hot-wire type gas flow meter as shown in Publication of Japanese Patent Application Laid-open No. Shou 56-18721, a bypass passage is formed independently from a main passage. In this case, a flow speed of intake air in the bypass passage is made different from a flow speed in the main passage by adjusting (increasing) the flow resistance of the bypass passage. Therefore, when the maximum flow rate is increased, the maximum flow speed in the bypass passage can be kept less than a desirable degree without increasing the size of the body forming the passages. But, in this structure, since the flow speed in the bypass passage is in proportion to the flow speed in the main passage, the flow speed in the bypass passage is decreased to less than a desirable degree when the flow speed in the main passage is decreased, so that the sensitivity of the hot wire is decreased and the generated noise (a small disorder in flow) is large in comparison with the overall flow.
In another conventional hot-wire type gas flow meter as shown in Publication of Japanese Utility Model Application Laid-open No. Shou 55-145321, a valve device is arranged in a bypass passage to control the flow resistance of the bypass passage for changing the flow rate of intake air. In this structure, the cross-sectional area of the bypass passage is increased to decrease the flow resistance characteristic so that the flow speed at a sensor portion (a hot wire) is increased when the flow rate is increased. This operation in this structure is opposite to an operation in the present invention. This operation is effective for compensating a variation in rate of an output voltage of the sensor to a flow speed at the sensor, because the output voltage of the sensor is in proportion to a square root of the flow speed at the sensor and the higher the flow rate is, the smaller is the rate of the output voltage of the sensor to the flow speed at the sensor.
In the conventional hot-wire type gas flow meters, a deterioration of the hot wire cannot be restrained when the maximum flow rate of intake air to be measured is large, and a suitable flow speed for preventing a decrease in sensitivity of the hot wire at the minimum flow rate cannot be obtained.